(1) Field of the Invention
This invention relates to a rare earth metal-transition metal-boron (R-T-B) permanent magnet with a high energy product and, in particular, to a method for producing such permanent magnets with anisotropy by sintering compact bodies of rapidly-quenched R-T-B alloy powder. (2) Description of the Prior Art
As an R-T-B permanent magnet alloy,, N. C. Koon and B. N. Das disclosed magnetic properties of amorphous and crystallized alloy of (Fe.sub.0.82 B.sub.0.18).sub.0.9 Tb.sub.0.05 La.sub.0.05 in Appl. Phys. Lett. 39(10) (1981), 840 (Reference 1). They wrote that crystallization of the alloy occurred near the relatively high temperature of 900 K., which also marked the onset of dramatic increase in the intrinsic coercive force. They found out that the alloy in the crystallized state appeared potentially useful as low cobalt permanent magnets.
J. J. Croat proposed amorphous R-Fe-B (Nd and/or Pr is especially used for R) alloy having magnetic properties for permanent magnets as disclosed in JP-A-59064739 (Reference 2, which is corresponding to U.S. patent application Serial Nos. 414936 and 508266) and JP-A-60009852 (Reference 3, which is corresponding to U.S. patent application Ser. Nos. 508266 and 544728). References 2 and 3 disclose to use other transition metal elements in place of or in part of Fe. Those magnetic properties were considered to be caused by a microstructure where Nd.sub.2 Fe.sub.14 B magnetic crystal grains having a grain size of 20-400 nm were dispersed within an amorphous Fe phase. Reference is further made to R. K. Mishra: J. Magnetism and Magnetic Materials 54-57 (1986) 450 (Reference 4).
The rapidly-quenched alloy ribbon is prepared by the continuous splat-quenching method which is disclosed in, for example, a paper entitled "Low-Field Magnetic Properties of Amorphous Alloys" written by Egami, Journal of the American Ceramic Society, Vol. 60, No. 3-4, March-April 1977, p.p. 128-133 (Reference 5). A similar continuous splat-quenching method is disclosed as a "Melt Spinning" method in References 2 and 3. That is, R-T-B molten alloy is ejected through a small orifice onto an outer peripheral chill surface of a copper disk rotating at a high speed. The molten alloy is rapidly quenched by the disk to form a rapidly-quenched ribbon. Then, a comparatively high cooling rate produces an amorphous alloy but a comparatively low cooling rate crystallises the metal.
According to References 2 and 3, the principal limiting factor for the rate of chill of a ribbon of alloy on the relatively cooler disc surface is its thickness. If the ribbon is too thick, the metal most remote from the chill surface will cool too slowly and crystallise in a magnetically soft state. If the alloy cools very quickly, the ribbon will have a microstructure that is somewhere between almost completely amorphous and very, very finely crystalline. That is, the slower cooling surface of the ribbon farthest from the chill surface is more crystallised but the other quickly cooling surface impinging the chill surface is hardly crystallised, so that crystallite size varies throughout the ribbon thickness.
References 2 and 3 describe that those magnetic materials exhibiting substantially uniform crystallite size across the thickness of the ribbon tend to exhibit better permanent magnetic properties than those showing substantial variation in crystallite size throughout the ribbon thickness.
In order to produce a practical magnet, the amorphous alloy is crushed and formed into a bonded magnet. Reference is made to a paper entitled "PROCESSING OF NEODYMIUM-IRON-BORON MELT-SPUN RIBBONS TO FULLY DENSE MAGNETS" presented by R. W. Lee et al at the International Magnetics Conference, held at St. Paul, Minn., on Apr. 29, 1985, and published in IEEE Transactions on Magnetics, Vol. MAG-21, No. 5, September 1985, Page 1958 (Reference 6).
Generally speaking, the amorphous alloy can provide only an isotropic magnet because of its crystallographically isotropy. This means that a high performance anisotropic permanent magnet cannot be obtained from the amorphous alloy. However, Reference 6 also discloses that magnetic alignment was strongly enhanced by upsetting fully dense hot-pressed samples of crushed amorphous alloy. But the technique cannot yet provide an anisotropic permanent magnet having a satisfactorily high energy product. For example, the hot-pressed magnet has a residual magnetic flux density Br of 7.9 kGauss, an intrinsic coercive force .sub.I H.sub.C of 16 kOe, and an energy product (BH)max of 13 MGOe.
JP-A-60089546 (Reference 7) discloses a rapidly quenched R-Fe-B permanent magnet alloy with a high coercive force. The alloy contains very fine composite structures less than 5 .mu.m predominant of tetragonal crystal compositions and is crushed into powders having particle sizes of -100 Tyler mesh (less than 300 .mu.m) to produce a bonded magnet. Although Reference 7 describes possibility of application of the crushed powders to a sintered magnet and a c-axis anisotrophy appreciated by application of X-ray diffraction microscopy to a surface of the alloy, no anisotropic sintered permanent magnet is disclosed. In practice, the crushed powder cannot be magnetically aligned and a sintered magnet therefore cannot be obtained with a high magnetic anisotropy.
Sagawa et al proposed an anisotropic R-Fe-B sintered magnet in JP-A-59046008 (Reference 8) which was produced from an ingot of an alloy of R (especially Nd), Fe, and B by a conventional powder metallurgical processes. The sintered magnet has more excellent magnetic properties for permanent magnets than the known Sm-Co magnets.
However, the R-Fe-B alloy tends to be oxidized in the production of the magnet, because the R-Fe-B alloy ingot comprises the magnetic crystalline phase of the chemical compound R.sub.2 Fe.sub.14 B and the R-rich solid solution phase and because the solid solution phase is very active to oxygen. Further, the solid solution phase is difficult to be uniformly ground into particles. Accordingly, it is difficult to produce an anti-corrosion anisotropic sintered magnet having a high energy product.
It is known in the prior art that the rapidly quenched R-T-B alloy ribbon is readily ground into powder having a small distribution of particle sizes and that is has a high corrosion resistance in comparison with the alloy ingot.